Fossils are the remains or evidence of ancient plants or animals that have been preserved in the rocks of the earth’s crust.Most fossils represent the preservable hard parts of some prehistoric organism that once lived in the area in which the remains were collected.
The wordfossilis derived from the Latin wordfossilis, meaning “dug up,” and for many years any unusual object dug out of the ground was considered to be a “fossil.” For this reason some of the earlier books dealing with fossils include discussions of rocks, minerals, and other inorganic objects.
There is much evidence to indicate that man has been interested in fossils since the very earliest times, andfossilshells, bones, and teeth have been found associated with the remains of primitive and prehistoric men. It is quite possible that the owners of these objects believed that they possessed supernatural powers, such as healing properties or the ability to remove curses.
During the earliest periods of recorded history, certain Greek scholars found the remains of fish and sea shells in desert and mountainous regions. These men were greatly puzzled by the occurrence of these objects at such great distances from the sea, and some of them devoted considerable time to an explanation of their presence.
In 450 B.C., Herodotus noticed fossils in the Egyptian desert and correctly concluded that the Mediterranean Sea had once been in that area.
Aristotle in 400 B.C. stated that fossils were organic in origin but that they were embedded in the rocks as a result of mysterious plastic forces at work within the earth. One of his students, Theophrastus (about 350 B.C.), also believed that fossils represented some form of life but thought that they had developed from seeds or eggs that had been planted in the rocks.
Strabo (about 63 B.C. to A.D. 20) was another important Greek scholar who attempted to explain the presence of fossils. He noted the occurrence of marine fossils well above sea level and correctly inferred that the rocks containing them had been subjected to considerable elevation.
During the “Dark Ages” fossils were alternately explained as freaks of nature, the remains of attempts at special creation, and devices of the devil which had been placed in the rocks to lead men astray. These superstitious beliefs and the opposition from religious authorities hindered the study of fossils for hundreds of years.
In approximately the middle of the fifteenth century the true origin of fossils was generally accepted, and they were considered to be the remains of prehistoric organisms which had been preserved in the earth’s crust. With the definite recognition of fossils as organic remains, many of the more primitive theories were discarded for one just as impractical—these remains were considered remnants of the Great Flood as recorded in the Scriptures. The resulting controversy between scientists and theologians lasted for about 300 years.
During the Renaissance several of the early natural scientists concerned themselves with investigations of fossils. Noteworthy among these was Leonardo da Vinci, the famous Italian artist, naturalist, and engineer. Leonardo insisted that the Flood could not be responsible for all fossils nor for their occurrence in the highest mountains. He reaffirmed the belief that fossils were indisputable evidence of ancient life, and that the sea had once covered northern Italy. Leonardo explained that the remains of the animals that had inhabited this ancient body of water were buried in the sediments of the sea floor, and that at some later date inearth history this ocean bottom was elevated well above sea level to form the Italian peninsula.
In the late eighteenth and early nineteenth centuries the study of fossils became firmly established as a science, and since that time fossils have become increasingly important to the geologist.
The study of fossils is calledpaleontology(Greekpalaios, ancient;ontos, a being;logos, word or discourse). Information gathered with the help of paleontology has greatly increased the knowledge of ancient plants and animals and of the world in which they lived.
Fossils represent the remains of such great numbers and various types of organisms that paleontologists have found it helpful to establish four main divisions within their science.
Paleobotany deals with the study offossilplants and the record of the changes which they have undergone.
This is the study offossilanimals without a backbone or spinal column. These include such forms as fossil protozoans (tiny one-celled animals), snails, clams, starfish, and worms, and usually represent the remains of animals that lived in prehistoric seas.
Because invertebrate remains are the most common fossils in Texas, this book is devoted largely to the discussion of invertebrate fossils and their method of collection.
Thevertebratepaleontologist studies the fossils of animals which possessed a backbone or spinal column. The remains of fish, amphibians, reptiles, birds, and mammals are typical vertebrate fossils.
Micropaleontology is the study of fossils that are so small that they are best studied under a microscope. These tiny remains are called microfossils and usually represent the shells or fragments of minute plants or animals. Because of their small size, microfossils can be brought out of wells without being damaged by the mechanics of drilling or coring. For this reason microfossils are particularly valuable to the petroleum geologist who uses them to identifyrockformations thousands of feet below the surface.
The majority of fossils are found in marinesedimentary rocks.These are rocks that were formed when salt-water sediments, such as limy muds, sands, or shell beds, were compressed and cemented together to form rocks. Only rarely do fossils occur in igneous and metamorphic rocks. Theigneous rockswere once hot and molten and had no life in them, andmetamorphic rockshave been so greatly changed or distorted that any fossils that were present in the originalrockhave usually been destroyed or so altered as to be of little use to the paleontologist.
But even in the sedimentary rocks only a minute fraction of prehistoric plants and animals have left any record of their existence. This is not difficult to understand in view of the rather rigorous requirements of fossilization.
Although a large number of factors ultimately determine whether an organism will be fossilized, the three basic requirements are:
1.The organism should possess hard parts.These might be shell, bone, teeth, or the woody tissue of plants. However, under very favorable conditions of preservation it is possible for even such fragile material as an insect or a jellyfish to become fossilized.
2.The organic remains must escape immediate destruction after death.If the body parts of an organism are crushed, decayed, or badly weathered, this may result in the alteration or complete destruction of thefossilrecord of that particular organism.
3.Rapid burial in a material capable of retarding decomposition.The type of material burying the remains usually depends upon where the organism lived. The remains of marine animals are common as fossils because they fall to the sea floor after death, and here they are covered by soft muds which will be the shales and limestones of later geologic periods. The finer sediments are less likely to damage the remains, and certain fine-grainedJurassiclimestones in Germany have faithfully preserved such delicate specimens as birds, insects, and jellyfishes.
Ash falling from nearby volcanoes has been known to cover entire forests, and some of thesefossilforests have been found with the trees still standing and in an excellent state of preservation.
Quicksand and tar are also commonly responsible for the rapid burial of animals. The tar acts as a trap to capture the beasts and as an antiseptic to retard the decomposition of their hard parts. The Rancho La Brea tar pit at Los Angeles, California, is famous for the large number offossilbones that have been recovered from it. These include such forms as the sabre-tooth cat, giant ground sloths, and other creatures that are now extinct. The remains of certain animals that lived during the Ice Ages have been incorporated into the ice or frozen ground, and some of these frozen remains are famous for their remarkable degree of preservation.
Although untold numbers of organisms have lived on the earth in past ages, only a minute fraction of these have left any record of their existence. Even if the basic requirements of fossilization have been fulfilled, there are still other reasons why some fossils may never be found.
For example, large numbers of fossils have been destroyed by erosion or their hard parts have been dissolved by underground waters. Others were entombed in rocks that were later subjected to great physical change, and fossils enclosed in these rocks are usually so damaged as to be unrecognizable.
Then, too, manyfossiliferousrocks cannot be studied because they are covered by water or great thicknesses of sediments,and still others are situated in places that are geographically inaccessible. These and many other problems confront the paleontologist as he attempts to catalog the plants and animals of the past.
The missing pages in thefossilrecord become more obvious and more numerous in the older rocks of the earth’s crust. This is because the more ancient rocks have had more time to be subjected to physical and chemical change or to be removed by erosion.
There are many different ways in which plants and animals may become fossilized. The method of preservation is usually dependent upon (1) the original composition of the organism, (2) where it lived, and (3) the forces that affected it after death.
Most paleontologists recognize four major types of preservation, each being based upon the composition of the remains or the changes which they have undergone.
This type offossilis formed only under very special conditions of preservation. To be preserved in this manner, the organism must be buried in a medium capable of retarding decomposition of the soft parts. Materials that have been known to produce this type of fossilization are frozen soil or ice, oil-saturated soils, andamber(fossil resin). It is also possible for organic remains to become so desiccated that a natural mummy is formed. This usually occurs only in arid or desert regions and when the remains have been protected from predators and scavengers.
Probably the best-known examples of preserved soft parts offossilanimals have been discovered in Alaska and Siberia. The frozen tundra of these areas has yielded the remains of large numbers of frozen mammoths—a type of extinct elephant (Pl. 49). Many of these huge beasts have been buried for as long as 25,000 years, and their bodies are exposed as the frozen earth begins to thaw. Some of these giant carcasses have been so well preserved that their flesh has been eaten by dogs and their tusks sold by ivory traders. Many museums display the original hair and skin of these elephants, and some have parts of the flesh and muscle preserved in alcohol.
Original soft parts have also been recovered from oil-saturated soils in eastern Poland. These deposits yielded the well-preserved nose-horn, a foreleg, and part of the skin of an extinct rhinoceros.
The natural mummies of ground sloths have been found in caves and volcanic craters in New Mexico and Arizona. The extremely dry desert atmosphere permitted thorough dehydration of the soft parts before decay set in, and specimens with portions of the original skin, hair, tendons, and claws have been discovered.
One of the more interesting and unusual types of fossilization is preservation inamber. This type of preservation was made possible when ancient insects were trapped in the sticky gum that exuded from certain coniferous trees. With the passing of time this resin hardened, leaving the insect encased in a tomb of amber, and some insects and spiders have been so well preserved that even fine hairs and muscle tissues may be studied under the microscope.
Although the preservation of original soft parts has produced some interesting and spectacular fossils, this type of fossilization is relatively rare, and the paleontologist must usually work with remains that have been preserved in stone.
Almost all plants and animals possess some type of hard parts which are capable of becoming fossilized. Such hard parts may consist of the shell material of clams, oysters, or snails, the teeth or bones of vertebrates, the exoskeletons of crabs, or the woody tissue of plants. These hard parts are composed of various minerals which are capable of resisting weathering and chemical action, and fossils of this sort are relatively common.
Many of thefossilmollusks found in theTertiaryandCretaceousrocks of Texas have been preserved in this manner. In some of the specimens the original shell material is so well preserved that the iridescent mother-of-pearl layer of the shell is found virtually intact. This type of preservation is less common, however, in the older rocks of the State.
PLATE 2Types ofFossilPreservation
PLATE 2Types ofFossilPreservation
At certain localities in north and central Texas the Woodbine sands of UpperCretaceousage (geologic time scale and geologic map, Pls.1,10) contain large numbers of shark and fish teeth (Pl. 37), fish scales and vertebrae. The remains of these vertebrates are unusually well preserved and are prized by both amateur and professional collectors.
Hard parts composed ofcalcite(calcium carbonate) are very common among the invertebrates. This is particularly true of the shells of clams, snails, and corals. Many of these shells have been preserved with little or no evidence of physical change (Pl. 2).
The bones and teeth of vertebrates and the exoskeletons of many invertebrates contain large amounts of calcium phosphate. Because this compound is particularly weather resistant, manyphosphaticremains (such as the fish teeth in the Woodbine sands) are found in an excellent state of preservation.
Many organisms having skeletal elements composed ofsilica(silicon dioxide) have been preserved with little observable change. Thesiliceoushard parts of many microfossils and certain types of sponges have become fossilized in this manner (Pl. 14).
Some organisms have anexoskeleton(outer body covering) composed ofchitin, a material that is similar to finger nails. The fossilizedchitinousexoskeletons of arthropods and other organisms are commonly preserved as thin films of carbon because of their chemical composition and method of burial.
The original hard parts of an organism normally undergo great change after burial. These changes take place in many ways, but the type of alteration is usually determined by the composition of the hard parts and where the organism lived. Some of the more common processes of alteration are discussed below.
This process, known also asdistillationtakes place as organic matter slowly decays after burial. During the process of decomposition, the organic matter gradually loses its gases and liquids leaving only a thin film of carbonaceous material (Pl. 2, fig. 7). This is the same process by which coal is formed, and large numbers of carbonized plant fossils have been found in many coal deposits.
In Texas the carbonized remains of plants, fish, and certain invertebrates have been preserved in this manner, and some of these carbon residues have accurately recorded even the most minute structures of these organisms.
Many fossils have been permineralized or petrified—literally turned to stone. This type of preservation occurs when mineral-bearing ground waters infiltrate porous bone, shell, or plant material. These underground waters deposit their mineral content in the empty spaces of the hard parts making them heavier and more resistant to weathering. Some of the more common minerals deposited in this manner arecalcite,silica, and various compounds of iron.
This type of preservation takes place when the original hard parts of organisms are removed after being dissolved by underground water. This is accompanied by almost simultaneous deposition of other substances in the resulting voids. Some replaced fossils will have the original structure destroyed by the replacing minerals.Others, as in the case of certain silicified tree trunks, may be preserved in minute detail.
Although more than 50 minerals have been known to replace original organic structures, the most frequent replacing substances arecalcite,dolomite(a calcium magnesium carbonate),silica, and certain iron compounds.
Calcareousreplacementoccurs when the hard parts of an organism are replaced bycalcite,dolomite, oraragonite(a mineral which is composed of calcium carbonate but which is less stable than calcite). The exoskeletons of many corals, echinoderms, brachiopods, and mollusks have been replaced in this manner.
When the original organic hard parts have been replaced bysilicathefossilis said to have undergonesilicification, and this type ofreplacementoften produces a very high degree of preservation. This is particularly true of the silicifiedPermian(geologic time scale,Pl. 1) fossils from the Glass Mountains in Brewster County. These fossils are embedded in limestone which must be dissolved in vats of acid, and after the enclosingrockhas been dissolved the residue yields an amazingvarietyof perfectly preserved invertebrate fossils (Pl. 3).
SilicifiedCretaceousfossils have been recovered from the Edwards limestone of central Texas. The silicifiedfaunais restricted to a few scattered localities, each of which may yield many unusually well-preserved fossils.
Several different iron compounds have been known to replace organic matter. Many Texas limestones containfossilsnails and clams which have had their original shell material replaced by iron compounds such as limonite, hematite, marcasite, orpyrite. Certain of thefossiliferousTertiarysandstones of the Texas Gulf Coast area contain large amounts ofglauconitewhich commonly replaced organic material.
In some areas entire faunas have been replaced by iron compounds. Such is the case in the famous “PyriteFossilZone” of the Pawpawformation(LowerCretaceous) in Tarrant County. The fossils in this part of the formation are very small or “dwarfed” and have been replaced by limonite, hematite, or pyrite. Ammonites, clams, snails, and corals are particularly abundant at this locality.
Fossils consist not only of plant and animal remains but of any evidence of their existence. In this type of fossilization there is no direct evidence of the original organism, rather there is some definite indication of the former presence of some ancient plant or animal. Objects of this sort normally furnish considerable information as to the identity or characteristics of the organism responsible for them.
Many shells, bones, leaves, and other forms of organic matter are preserved as molds and casts. If a shell had been pressed down into the ocean bottom before thesedimenthad hardened intorock, it may have left the impression of the exterior of the shell. This impression is known as amold(Pl. 2). If at some later time this mold was filled with another material, this produced acast. This cast will show the original external characteristics of the shell. Such objects are calledexternal moldsif they show the external features of the hard parts (Pl. 2, fig. 2) andinternal molds(Pl. 2, fig. 3) if the nature of the inner parts is shown.
Molds and casts are to be found in almost all of the fossil-bearing rocks of Texas, and they make up a large part of mostfossilcollections. It is particularly common to find fossil clams and snails preserved by this method. This is primarily because their shells are composed of minerals that are relatively easy to dissolve, and the original shell material is often destroyed.
PLATE 3Silicified BrachiopodsAll specimens fromPermianlimestones of the Glass Mountains, Brewster County, Texas
PLATE 3Silicified Brachiopods
All specimens fromPermianlimestones of the Glass Mountains, Brewster County, Texas
Many animals have left records of their movements over dry land or the sea bottom. Some of these, such as footprints (Pl. 4), indicate not only the type of animal that left them but often provide valuable information about the animal’s environment.
Thus, the study of aseriesof dinosaur tracks would not only indicate the size and shape of the foot but also provide some information as to the weight and length of the animal. In addition, the type ofrockcontaining the track would help determine the conditions under which the dinosaur lived.
Some of the world’s most famous dinosaur tracks are to be found in the LowerCretaceouslimestones in Somervell County, Texas. These footprints, which are about 110,000,000 years old (Pl. 4), were discovered in the bed of Paluxy Creek near the town of Glen Rose. Large segments of therockcontaining these tracks were collected by paleontologists of the American Museum of Natural History in New York City and the Texas Memorial Museum at Austin. Great slabs of limestone were transported to the museums, replaced in their original position, and are now on display as mute evidence of the gigantic size of these tremendous reptiles.
Invertebrates also leave tracks and trails of their activities, and these markings may be seen on the surfaces of many sandstone and limestone deposits. These may be simple tracks, left as the animal moved over the surface, or the burrows of crabs or other burrowing animals. Markings of this sort provide some evidence of the manner of locomotion of these organisms and of the type of environment that they inhabited.
Coprolites arefossildung or body waste (fig. 1). These objects can provide valuable information as to the food habits or anatomical structure of the animal that made them.
Fig. 1.Sketch of acoprolite—fossilized animal excrement.
Fig. 1.Sketch of acoprolite—fossilized animal excrement.
These highly polished well-rounded stones (fig. 2) are believed to have been used in the stomachs of reptiles for grinding the food into smaller pieces. Large numbers of these “stomach stones” have been found with the remains of certain types of dinosaurs.
Fig. 2.Sketch of agastrolith—the gizzard stone of an ancient reptile.
Fig. 2.Sketch of agastrolith—the gizzard stone of an ancient reptile.
Among the many inorganic objects formed by nature there are some that bear superficial resemblance to plants or animals. Because they are often mistaken for organic remains, these objects have been calledpseudofossils, or “false fossils.”
Fig. 3.Dendrites. These thin branching mineral deposits bear a marked resemblance to plants, hence they are called pseudofossils.
Fig. 3.Dendrites. These thin branching mineral deposits bear a marked resemblance to plants, hence they are called pseudofossils.
Although these closely resemble the remains of ferns or other plant material (fig. 3), dendrites are actually thin incrustations of manganese dioxide. They are often found along the bedding planes ofCretaceousandPaleozoic(geologic time scale,Pl. 1) limestones in many parts of Texas.
Plate 4Dinosaur tracks in limestone in bed of Paluxy Creek near Glen Rose, Somervell County, Texas.Photograph courtesy of the American Museum of Natural History.Permission to reproduce by R. T. Bird.
Plate 4Dinosaur tracks in limestone in bed of Paluxy Creek near Glen Rose, Somervell County, Texas.Photograph courtesy of the American Museum of Natural History.Permission to reproduce by R. T. Bird.
These are striations that are produced whenrocksurfaces move past each other while being fractured.Slickensidesmay superficially resemble certain of thePennsylvaniancoal plants of Texas.
Sinceslickensidesare commonly at an angle to the bedding plane and plant remains lie parallel to the bedding plane, the two are usually easily distinguished.
Many shales and sandstones contain hardened masses of minerals androckthat are often mistaken for fossils. These masses, called concretions, are usually found weathered out of the surrounding rock and may assume the shape of bones, flowers, vegetables, turtles, etc. Although these concretions do not represent organic remains, it is sometimes possible to find true fossils inside them.
Infossilcollecting, as in most “collecting” hobbies, the key to success lies in knowing where to look, what equipment to use, and the most effective methods of collecting.
Fossilcollecting is a relatively inexpensive hobby because it requires a minimum of supplies and equipment. However, as in almost any hobby, there are certain basic items of equipment that must be acquired.
The hammer is the basic tool in the collector’s kit. Almost any type of hammer is satisfactory, but as collecting experience is gained it may be desirable to get a geologist’s hammer. These hammers, also called mineralogist’s or prospector’s picks, are of two types. One type has a square head on one end and a pick on the other (Pl. 5): the other type is similar to a stonemason’s or bricklayer’s hammer and has a chisel end instead of the pointed pick end. The square head of the hammer is useful in breaking or chipping harder rocks, and the chisel or pick end is good for digging, prying, and splitting soft rocks.
It will be necessary to have some type of bag in which to carry equipment, fossils, and other supplies. A Boy Scout knapsack, musette bag (Pl. 5), hunting bag, or similar canvas or leather bag is suitable.
A pair of chisels is useful when fossils must be chipped out of the surroundingrock. Two sizes, preferably ½ and 1 inch, will usually suffice. A small sharp punch or awl is effective in removing smaller specimens from the softer rocks.
Some specimens are more fragile than others, and these should be handled with special care. Several sheets of newspaper should always be kept in the collecting bag, and each specimen should be wrapped individually as it is collected. Such precautions taken in the field will usually prevent prized specimens from being broken or otherwise damaged. In addition to newspaper, it is wise to carry a supply of tissue paper in which to wrap more fragile specimens.
It is most important to have some method of recording where the fossils were found. It is very easy to forget where the material was collected, and one shouldneverrely on memory. A small pocket-sized notebook is inexpensive and just the right size to carry in the field.
A highway or county map should be used to find the geographic location of each collecting locality. Maps of Texas counties can be obtained from the Texas Highway Department, File D-10, Austin 14, Texas. These maps come in three different sizes, but for most purposes the 18×25-inch sheets, with a scale of ½ inch = 1 mile, will be satisfactory. These are available for all counties and may be purchased at a nominal price.
A magnifying glass or hand lens (Pl. 5) is useful for looking at small specimens and will also prove helpful in examining the finer details of larger fossils. A 10-power magnification is satisfactory for most purposes, and several inexpensive models are available.
Small bags are useful in separating specimens from different localities. Heavy-duty hardware bags for large rough material and medium-weight grocery bags for smaller specimens may be used. Locality data may be written directly on the bag or on a label placed inside with the fossils. As an added precaution some collectors do both. The more serious collector may want to use a cloth geological sample bag (Pl. 5).